How do monsters change the color of Frisk (and also Kris') SOUL?

Undertale Discussion
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mostly related to the Undertale characters, but i'm also curious about the characters in Deltarune too.

i looked around the wiki and other blogs and i just have more questions so i'm asking here.

also kinda spoilers for chapter 4 here, so for safety i'll just blur out the rest of the post

is it like a superpower? i assume they learn it, as we all assume Undyne learns it from Gerson as per Deltarune, but again, likely a different universe, so we don't know if Undertale's Gerson is able to change the SOUL color either. but if Gerson taught Undyne, does that mean there's levels to the powers? because Gerson is able to make the shield of the green SOUL more sides than just 4, and Undyne is not shown performing that power at all.

and it isn't like every monster can do it. in undertale, we only see Undyne, Muffet, Papyrus, Sans, and Mettaton do it. which those characters raise more questions. how come only the skeleton brothers know how to change the SOUL to blue? and if Mettaton is programmed with the yellow SOUL ability, who is to blame for giving that power to the robot, the ghost that was Mettaton before the suit, or Alphys? if Alphys, does Alphys know how to change the SOUL color? if the ghost, does Napstablook know by association, if both Sans and Papyrus do by association?

and when looking at Deltarune, how does Spamton NEO know?

i also remembered the SOULs have meanings, of course. but i can't find a way to link Muffet to perseverance and Mettaton to justice. maybe integrity for Sans, and kindess for Undyne i understand, i don't know it doesn't feel like that's the answer at all.

EDIT: forgot the phone gave Frisk the yellow powers, haven't played in years. regardless, how does the phone have that power? doesn't change the answer to my question.

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The phone gave frisk yellow soul
Meanwhile in spamton neo the soul just decided “rotation” and did that

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ohhh forgot about the phone, haven't played undertale in years

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I think it's just a gameplay element and there's not much more to it lorewise.

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correction: magic

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paint

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Undyne uses her spear, Muffet does her tea thing, Papyrus literally just goes “You’re blue now” and Sans can control gravity with his arms so that’s likely also how he turned the soul blue

I think I got everything that wasn’t mentioned already

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Gerson also uses his hammer in Deltarune and the soul kinda just does that in the spamton neo fight

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Well in Neo it’s bc spamton is about to kill everyone so the soul does that to save them

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I have wondered about this, too. I just assumed it was due to some kind of determination/soul interaction between the human target and the monster attacker until the SOUL just pulls yellow mode out in spamton NEO and that clearly was not spamton inducing it. there could be oodles of speculative bioligy that goes into this.

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I mean papyrus does say "YOUR'E BLUEY NOW BITCH" straight to your face

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M A G I C

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Warning! (crossed out P)Bluey levels rising!

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The yellow SOUL was shooting Mettaton, wasn't it?

I could be wrong, but I’m pretty certain the phone would simply fire a projectile at Mettaton; with the yellow being nothing more than a visual asset.

It's been a quite a bit on my end since playing Undertale, but I think the game supports this conclusion.